In a rolling traction variator, drive is transmitted through at least one roller (and more typically a set of rollers) running upon at least one a pair of rotary races. To provide traction between rollers and races they must be biased into engagement with each other. The biasing force is referred to herein as the “traction load”. In known variators the rollers and races do not make contact with each other since they are separated by a thin film of “traction fluid”. It is shear of this fluid which, given suitably high pressure, provides the requisite roller/race traction.
Control of traction load is important to variator performance. One reason for this is that energy losses taking place at the roller/race interface vary with traction load, which thus has a bearing on variator efficiency. These losses are due to (1) spin at the interface—i.e. rotation of one surface relative to the other, due to the fact that the two surfaces are following circular paths about different axes and (2) shear at the interface—i.e. speed difference between the two surfaces, producing the shear in the fluid. It is found that excessively high traction loads increase spin, losses while low traction loads lead to high shear losses, optimal efficiency lying between the two extremes.
Another reason why traction load control is important is that excessive slip of rollers relative to the races, in response to inadequate traction loading, can result in failure of the variator and damage to it.
It is known to vary traction load in sympathy with “reaction force”. To explain first of all what reaction force is, consider that due to the torque being transmitted the races exert a tangential force upon each of the rollers. This force must be reacted back to the transmission casing. In known rolling traction variators the rollers are typically movably mounted and the force exerted by the races is opposed by, and reacted to the casing through, an actuator acting upon the roller's mountings. The reaction force applied by the actuator is adjustable for the purpose of controlling the variator and is equal but opposite to the tangential force exerted by the races.
The variator's traction coefficient μ can be defined as follows
  μ  =      RF    TL  where TL is the traction load and RF is the reaction force. This is strictly a simplification, since the true coefficient of traction at any chosen roller/race interface depends upon the magnitude of the forces perpendicular and parallel to the interface, and the traction load is not generally perpendicular to the interface. However this simple definition will suffice for the present discussion.
Variators are known in which traction load is varied along with reaction force to provide a constant traction coefficient. Reference is directed in this regard to Torotrak's European patent EP 894210 wherein reaction force is provided by double acting hydraulic roller actuators and the two pressures at these actuators are also led to a hydraulic traction load actuator. The hydraulic coupling of roller and traction load actuators is advantageous because it allows the traction load to be very quickly varied along with reaction force. This is important in responding to “torque spikes”—rapid fluctuations in transmission torque occurring for example upon emergency braking of the vehicle. A torque spike produces a rapid change in reaction force which could lead to slip between rollers and races, were if not for the fact that, in the known arrangement, increased pressures which are created in the roller actuators are passed on to the traction load actuator to correspondingly increase traction load with little time lag.
To increase still further the speed of response of the traction load to the reaction force, Torotrak's International Patent Application PCT/GB02/01551, published under No. WO 02/079675, teaches how pressure from the roller actuators can be used to control a pilot operated valve which in its turn controls application of fluid from a high pressure source to the traction load actuator. The same document recognises the desirability of adjusting the traction coefficient and provides some ways in which this can be achieved.
One reason for adjusting the traction coefficient (as opposed to maintaining, so far as possible, a constant ratio of traction load to reaction force) is that the properties of the traction fluid, and consequently the appropriate traction coefficient, vary with temperature. It is also desirable to adjust μ with variator rolling speed and with variator ratio.
WO 02/079675 suggests that traction coefficient adjustment can be carried out by applying an adjustable force to the spool of the pilot operated valve using a solenoid. The effect is to add an offset to the traction load so that
  TL  =            RF      μ        -    OF  where μ is the traction coefficient which would be obtained without the solenoid force and OF is the offset produced by the solenoid force. It will be apparent that the ratio of traction load to reaction force varies as the magnitude of the reaction force varies and this is undesirable, particularly because an offset which produces an appropriate traction coefficient at high reaction force/traction load produces, at much lower levels of reaction force, a large error in traction load. Inaccuracy in this large offset could, furthermore, potentially result in the traction load being too small when the reaction force is low.
Rather than adding an offset to the traction load, it would be desirable to provide for adjustment of the traction coefficient itself, so that:—
  TL  =      RF          (              μ        +        K            )      where μ is once more the traction coefficient which would be provided in the absence of the adjustment and K is an adjustment determined by the control electronics associated with the variator. Given this relationship, changes in reaction force RF do not produce discrepancies in traction load. The desirability of this type of traction load control was recognised in WO 02/079675 but devising a practical hydraulic arrangement for achieving the relationship is problematic. That document did show one possible circuit which used a series pair of flow restrictors, one of which was variable, in a manner analogous to a potential divider in an electrical circuit, to modify the traction load pressure. This arrangement introduces certain problems of its own, particularly as it relies on a continuous flow of pressurized fluid out of the hydraulics, adding to the burden placed upon the associated pump or pumps.
The desirability of adjusting traction coefficient has also been recognised in U.S. Pat. No. 6,162,144, assigned to General Motors Corporation. However the circuit drawn in that patent simply uses a pulse width modulated valve to feed a percentage of the end load pressure to a second chamber of the traction load actuator, working in opposition to the main traction load pressure, to thereby adjustably modify the reaction load. It is believed that this would not provide a practical system capable of reacting with sufficient speed to rapid reaction force changes, the bandwidth of the pulse width modulated valve being too low.